Elongate composite structural members, such as spars, ribs, stringers and the like are typically used to provide structural support in a structure on a global or local scale. Spars and ribs for example provide the main structural framework for an aerofoil box or fuselage structure. Stringers for example are used as stiffening members.
Elongate composite structural members thus have portions that are adapted to abut another component, for example to stiffen, strengthen and/or to support such another component. The component against which the elongate composite structural abuts may for example be in the form of a panel or section of skin that defines a surface of an aircraft.
Such elongate composite structural members may have a cross-section in the form of a U-shape, in the form of a T-shape, in the form of an L-shape or other suitable shapes. Typically, the structural member will have a foot which has a shape adapted to abut the surface of the structure/component to be stiffened/supported and a web that projects from the foot and away from the surface of the structure/component to be stiffened/supported, the web increasing the stiffness/strength of the structural member. The web is sometimes referred to as the blade of the structural member.
The thickness or geometry of the surface of the structure/component to be stiffened/supported may vary, thereby producing local features in the face of the structure/component adjacent to the structural member. Thus, corresponding variations in the geometry of the structural member may be necessary. Local variations in the geometry of the structural member can however introduce manufacturing problems when fabricating composite structural members. For example, in order to increase the local strength or stiffness of an aircraft wing panel, it is common practice to vary the thickness of the panel locally where extra stiffness or strength is required. This results in pad-ups in the panel when viewed from the interior of the wing-box. Thus, the thickness of the panel, may, with increasing direction along the length of an associated spar or stringer, ramp up to a locally thicker section and then ramp down to a thinner section. To accommodate the change in thickness in the panel, the foot of the associated spar/stringer needs to correspondingly ramp up and ramp down. The shape of the spar/stringer may therefore include local variations in its cross-sectional geometry, as a function of distance along its length.
The desired shape of structural member for use when stiffening a panel may therefore be complicated and may deviate from a linearly symmetrical geometry. Manufacturing composite structural members having a complicated geometry can be difficult. If local changes in cross-sectional geometry of the structural member are required, defects may be introduced during the fabrication process. Such defects usually result from layers of fibre material being compressed or folded in regions where there is too much material in view of the local geometry. This can produce creases in the final product, typically in the form of transverse creases. Defects can also result from layers of fibre material being stretched and/or stressed in regions where there is too little material in view of the local geometry. This too can produce creases in the final product, typically in the form of longitudinal creases. Either of the foregoing types of defect (too little material or too much material) can result in undesirable weakening of, and/or localised internal stresses in, the composite material in such regions. Such defects are typically allowed for and suitable margins built in by adding extra material in such regions, to counteract the strength-reducing defects. Whilst the strength of the resulting component may not be thus compromised, this technique introduces a weight penalty and excessive structural volume.
The present invention seeks to mitigate one or more of the above-mentioned problems. Alternatively or additionally, the present invention seeks to provide an improved shape of elongate composite structural member and/or an improved method of designing and/or manufacturing the same.